A. Field of the Invention
The device and method of the present invention relate to devices for providing capacitance in electrical circuits, commonly referred to as capacitors, and, more particularly, to devices having an elongated, generally tubular shape for providing a variable amount of capacitance in electrical circuits, and commonly referred to as piston trimmers.
B. Description of the Prior Art
A piston trimmer is well known in the prior art and consists of a variable capacitor having an elongated, generally tubular shape and including a pair of capacitive plates formed by a pair of terminals spaced at opposite ends of a tubular dielectric member or core. The trimmer typically includes an elongated conductive member, such as a conductive screw, positioned within the core, electrically connected to one of the pair of terminals and variably axially movable within the core to vary the capacitance value of the trimmer.
A particular, widely used, prior art piston trimmer includes a pair of conductive terminals spaced at opposite ends of a tubularly shaped polystyrene core. A conductive screw is variably positioned within the polystyrene core. By varying the axial position of the conductive screw within the polystyrene core, the capacitance value of the trimmer is varied.
A major disadvantage of this type of piston trimmer is the unreliable and inaccurate capacitance value of the trimmer. This disadvantage is believed to result from the presence of air voids between the tips of the threads of the conductive screw and the inner surface of the polystyrene core. The presence of air voids varies the value of the dielectric constant of the dielectric medium, that is, the polystyrene core, separating the tips of the threads of the conductive screw and one of the pair of conductive terminals. The air voids may result from manufacturing tolerances and may be present upon the initial assembly of the piston trimmer. Additionally, the air voids may result from the relatively high temperatures, typically in the range of from 380.degree.F to 430.degree.F, encountered by the trimmer during a soldering operation, such as the rather common soldering operation of attaching the terminals of such a trimmer to a printed circuit board. These temperatures may cause the polystyrene core to plasticize or melt, resulting in the formation of the above-mentioned air voids and, possibly, in the formation of air voids between the mating surfaces of the polystyrene core and the conductive terminals which, in addition to altering the above-mentioned dielectric constant, may result in the separation of one or more of the conductive terminals from the polystyrene core.